Process controller

ABSTRACT

A process control apparatus detachably packaged in two integral sections. The lower section contains a compartment for a reference solution and another compartment for a process solution; the two compartments being in thermal equilibrium and having an electrical bridge therebetween. A pair of specific ion sensors extend downwardly from the upper section and reach into the two solution compartments when the controller is assembled. In operation a process solution flows through the controller and the two specific ion sensors generate a potential difference signal for operation of a solenoid valve which controls addition of a concentration correcting fluid to the process stream. There is disclosed an application of such a controller to a three stage washing system for papermaking pulp.

United States Paten 1191 Kelch l- 1451 Nov. 6, 1973 v [5 1 PROCESSCONTROLLER I 3,306,837 2/1967 Riseman et al. 204 195 0 3,539,455 11/1970Clark 204/l95 P [75] lmemors' ii g t; 3,563,874 2/1971 Ross et al 204195 M Cedarv1lle, all of Oh1o Primary Tung [73] Assignee: The MeadCorporation, Dayton, Attorney-John Donahue Ohio 322 Filed: Mar. 16',1972. [57] ABSTRACT A process control apparatus detachably packaged 1nPP ,116 two integral sections. The lower section contains a compartmentfor a reference solution and another [52] us. 01 204/195 M, 204/1 T,204/195 R, mmpartmem alpmcelss "9 137/93 ments being in thermalequilibnurn and hav ng an elec- 511 161.01. G01n 27/46 ical bndgetherebetween A spec'fic [58] Field of Search 204/1 T, 195 R, 195 0,extend dwnwardly the and 2O4/195 M reach into the two solutioncompartments when the controller is assembled. In operation a processsolution [56] References Cited flows through the controllpcr1 and thetwo slpfecific ion sensors enerate a otent1a 1 erence slgna or opera,-UNITED STATES PATENTS tion of g solenoid valve which controls additionof a 3,434,953 3/1 969 Porter et al 204/195 G concentration oj-r tingfluid to the process stream. g 5 There is disclosed an application ofsuch a controller to 3:696:0l9 10 1972 Ali s l s: :1 51 1. I: 204l195 oa three stage washing system for Papermaking pulp 3,479,255 14 Claims,12 Drawing Figures ll/l969 Arthur..... 204/1 T I I I 1 1 1 I 1 ...I l lI I 1 1 l l I L.

PATENTED NOV 6 I973 SHEET 7 BF 7 FIG. 11

FIG. 12

success coursosrnn CROSS REFERENCE TO RELATED APPLHGATION Thisapplication relates to U. S. Pat. Application Ser. No. 235,055, nowabandoned, filed on even date herewith.

BACKGROUND or "run rNvENrroN This invention relates to the field'ofautomatic process control. More particularly it concernsithe continu ousreplenishment of chemical solutions employed in a wide variety ofprocesses. Typical processes wherein the invention may find applicationare wood pulp bleaching, vegetable washing and sterilizing, photographicdeveloping, electroplating, and coating. In each of these cases there isemployed a. working solution which may become depleted of a particularchemical over a period of time. If the process is to proceed withoutinterruption, then it becomes necessary to make continuous measurementsof solution concentration and replenish the solution with the depletedchemical in response thereto.

Other applications of the invention are in processes wherein there maynot be a working solution as such, but wherein there is nevertheless asolution requiring continuous chemical monitoring and control. Waterchlorination and sewage treatment are examples of applications of thistype. Some typical prior art devices for accomplishing continuousautomatic replenishment control are disclosed in Russell U. S. Pat. No.3,195,551, Ladd U. S. Pat. No. 3,273,580, Cardeiro U. S. Pat. No.3,440,525 and Schumacher U. S. Pat. No. 3,529,529. Other earlier devicesrelated to the apparatus of this invention are disclosed in DataCorporation 7 SUMMARY OF THE INVENTION This invention overcomes theinherent limitations of prior art process controllers by providing acompact easily assembled device having a pair of specific ion sensorsextending into two chambers; one of which contains a reference solutionand the othera process solution. Provision is made for continuous flowof the process stream through the controller, and for continuousgeneration of a concentration correction signal based upon the voltageoutput difference between the two specific ion sensors. The correctionsignal preferably operates a solenoid valve which admits concentrationcorrecting fluid into the process stream at a point upstream from thecontroller. In preferred embodiment the controller is made in twodetachable sections and is provided with means for adjusting theresponse time of the controller to match the response time of theprocess being controlled.

BRIEF DESCRlPTlON OF THE DRAWlNGS FIG. 1 is a schematic representationin crossection of a preferred embodiment of a process controller made inaccordance'with this invention.

FIG. 2 is a left frontal view of an assembled process controller.

FIG. 3 is a right rear view of an assembled process controller.

FIG. 4 is a view of an assembled upper section for a process controller.

FIG. 5 is a partially cut away plan view ofan assembled lower sectionfor a process controller. 7

FIG. 6 is a crossection taken along line 6-6 of FIG.

FIG. 7 is a plan view of a partially cut away bottom plate for theprocess controller of H65. 1 throughd.

H6. 8 is a schematic representation of the fluid flow system for theprocess controller of FlGS. 1 through 6.

H6. 9 is a schematic drawing of a three stage pulp washing systememploying a process controller in accordance with this invention.

FIG. 10 is a schematic diagram of the process controller electronics.

FIG. 11 is a schematic representation of a process controller employingthis invention in alternate embodiment.

FIG. 12 shows another alternate embodiment of the invention.

DESCRllPTlON OF THE FREFERRED EMBODIMENTS A preferred embodiment of theinvention is illustrated schematically in FIG. 1 wherein is shown aprocess controller H comprising a lower section 2 and an upper section3. A process solution 9 flows through the base of the lower section 2and a portion thereof is admitted upwardly into a measuring chamber viaan admission channel lll. Process solution 9 rises within the measuringchamber until the level thereof reaches an opening at the top of standpipe 8. From there it runs downwardly to join the main stream. Exitchannel 12 is provided for this'purpose.

A reference tube 10 tits in a well at the base of lower section 2 andextends upwardly to upper section 3. Reference tube 10 is tapered by arough grind around the lower end thereof for mating reception by taperedwell wall 13. This provides a capillary junction for apurpose ashereinafter explained. The upper end of reference tube 10 fits withingroove M of upper section 3. A reference solution 7 is contained withinreference tube ill. Reference solution 7 is in electrical contact withprocess solution 9 by capillary contact along the interface of taperedwall 113 with the tapered lower end of reference tube 10. A referenceprobe 4 extends downwardly from upper section 3 into electrical contactwith reference solution 7. Also extending downwardly from upper section3 are a signal probe 5 and a ground rod 6. Signal probe 5 and ground rod(6 are both in electrical contact with process solution 9 in the chambersurrounding reference tube lil, Electrical resistance at the capillaryjunction may be reduced by providing apertures 77 (FIG. 6) at the lowerend of tube lid.

Reference solution 7 is preferably a sample of process solution obtainedunder ideal concentration conditions. Signal probe 5 and reference probe'8 contain specific ion sensors of identical construction. Each probegenerates a voltage related to the concentration of a particular ionpresent in the respective surrounding solution. The voltage outputs fromprobes and 5 are delivered to a difference amplifier packaged withinupper section 3. The output from this difference amplitier controls theopening and closing of a solenoid valve which regulates the flow of areplenishment solution into the process stream.

The ion sensitive element within probes 4 and 5 is a solid statemembrane of composition depending upon tye type of ion to be sensed. Forinstance, if it is desired to sense the presence of chloride ions, themembrane may be silver sulfide and silver chloride composition. Theconstruction and operation of such probes is well known as shown forinstance in Ross et al U. S. Pat. No. 3,563,874. The above mentionedSer. No. 235,055 provides another example of a probe well suited forsuch use. It is to be noted that while specific ion sensing probes arecommonly used to make electrochemical measurements, they are ordinarilyused in combination with an accurately calibrated reference electrodesuch as a calomel probe. This creates a requirement for corrections toeliminate a number of measuring errors well known in the electrochemicalart. In contrast thereto, this invention relies upon a null pointpotentiometry principle; no absolute concentration measurement beingmade. By using a symmetrical measuring configuration most significantsources of error are removed. It has been found, however, that certainother unexpected errors may arise in devices of this type.

One of these errors is due to evaporation from the reference solution.Even in a tightly sealed controller, water may evaporate from thereference solution overa long period of time, and condense on the undersurface of upper section 3. Thereafter it may drip down into the processsolution surrounding reference tube and leave the reference solutionwith an increased ion concentration. This source of error is removed byupward extension of reference tube 10 into channel 14 on the lowersurface of upper section 3. This restricts the above mentionedevaporation and condensation process to the confines of the referencetube. Water which evaporatesfrom the reference solution 7 tends tocollect on that portion of the under surface of upper section 3 which isdirectly above reference tube 10. Consequently the vapor whichthereafter condenses drips back inside reference tube 10. It is to benoted, however, that reference tube 10 does not fit tightly againstupper section 3. There is sufficient clearance around groove 14 topermit equalization of air pressure on both sides of the reference tube.

The other unexpected error is due to air bubbles curely in place. Probes4 and 5 are threaded at their upper ends for easy attachment to cap 42.Probes 4 and 5 each also have a pin connector at their upper ends fordetachable connection to circuit boards mounted within upper section 3.

It will be appreciated that the solid state membrane 39 installed at theend of each probe is adapted for sensing of only one specific ion. If itis desired to employ the process controller for replenishment of avariety of process solutions, then it is necessary only to provide' aset of probes for each type of solution. Upon completion of a processcontrol operation, clips 15 are extended and upper section 3 is pulledaway from lower section 2. Probes 4 and 5 are unscrewed from their seatsand a new set of probes screwed into place. Then the lower section isdrained, and a sample of the new solution at the appropriateconcentration is poured into reference tube 10. Following this, theupper section 3 is again lowered into lower section 2 and clamped inplace with rubber ring 33 providing a pressure seal as mentioned above.The lower extension of cap 42 fits snugly inside top plate 43 as shownin FIG. 6.

FIG. 5 is a partially cut away plan view of a fully assembled lowersection. The locations for reception of probes 4 and 5 and ground rod 6are shown in phantom lines thereon.

FIG. 6 shows a crossection of a fully assembled process controller takenalong line 6-6 of FIG. 5. All major structures are made of polyvinylchloride or other hard plastic material and are bonded together withresin cement. Two boards of electrical components are mounted insideupper section 3 as shown in FIG. 6. These are an amplifier board 25 anda power supply board 26. Receptacles 27 are provided on amplifier board25 for reception of the ground rod and the reference and signal probepins.

FIG. 8 provides a schematic representation of fluid flow routing fromconnector 22 to connector 23. The

' major portion of the fluid is admitted by a valve 24 into which tendto collect on the lower surface of signal I probe 5. These air bubblesinterfere with the migration of ions across the solid state membranethereby producing an erroneous output voltage. It has been found thatsuch bubble collection may be avoided by placing admission channel 11directly below signal probe 5.

Apparatus for practicing this invention may be conveniently packaged asshown generally in FIG. 2. Again, the numeral 1 denotes the processcontroller, numeral 2 the lower section thereof, and numeral 3 the uppersection thereof. Upper section 3 is releasably joined to lower section2.by means of a pair of clips 15, one on each side; A rubber ring 33(see FIGS. 4 and 6) creates a pressure seal between the upper and lowersections. Process solution enters at the rear of the controller (seeFIG. 3) via fluid connector 22 and leaves via fluid connector 23.

FIG. 4 shows the general exterior arrangement of a bypass channel 37 andflows directly out of the controller. A relatively small portion of theprocess solution is admitted into a sampling channel 34 and flowsforwardly to flow meter 16.

Flow meter 16 is a flow device of the common cone and ball type.Accordingly ball 17 sits at the lower end of meter 16 and is urgedupwardly in accordance with the rate of fluid flow. Needle valve 21provides fine adjustment of the upward fluid flow rate through flowmeter 16, and post 18 (FIG. 2) protects the meter against breakage. Thepurpose of flow rate adjustment is to match the response time of theprocess controller to that of the process being controlled. Suchresponse time matching is necessary for achieving dynamic stability insome applications.

After the process fluid sample reaches the top of flow meter 16 it isdirected laterally by channel 35 to connect with tube 19. Tube 19carries the process solution sample downwardly to channel 36 which thenleads rearwardly to aperture 38. The fluid flows upwardly throughaperture 38 filling cylindrical section 20 and overflowing downwardlythrough stand pipe 8. Stand pipe 8 empties into bypass channel 37.

FIGS. 5 and 7 show the details'of the above described fluid passages. Asillustrated in phantom lines on FIG.

7, the flow dividing action of valve 24 is accomplished drilled passagesin bottom plate 44. Aperture 28 is provided to enable fluid passage fromchannel 34 to flow meter 16. Aperture 29 enables fluid passage from tube19 to channel 36, and aperture 30 enables fluid passage from stand pipe8 to channel 37. A groove 31 is cut into the upper surface of bottomplate 44 to receive cylindrical section 20.

FIG. presents an electrical schematic for the process controller. Asshown therein, a volt DC power supply 46, a solid state relay 45, andassociated resistors, filter capacitors, and indicator lamps are mountedon circuit board 26. Circuit board 25 contains operational amplifiers53, 54, and 55 and driver amplifier 56. Other associated components area power switch 52 mounted on the front face of top section 3, a nullcontrol 47 also mounted on the front face of top section 3, and a fuse51 and connector 48 mounted at the rear of top section 3. In operationoutput voltages from probes 4 and 5 are delivered via their associatedconnectors 27 to amplifiers 53 and 54. Amplifier 55 operates as adifferential amplifier, and produces an output signal which is theamplified difference between the output signals of amplifiers 53 and 54.Capacitors 68 and 69 function as a broad band filter to reduce noisewhich otherwise would be present at the output of amplifier 55. Theoutput from amplifier 55 is fed to transistor 56 which operates as adriver amplifier for solid state relay 45. Relay 45 contains an SCRwhich is fired by current flow on line 66.

When relay 45 closes, current from power supply line 69 is routed torelay output line 68, and then through switch 52 and connector 48 to asolenoid valve such as valve 111 of FIG. 9. Thisinitiates a flow ofmakeup or correcting solution, and also completes a circuit foractivating a neon lamp 49, the ON indicator. The controller is designedto operate on an ON/OFF basis, and neon lamp 50 indicates the OFFcondition. When relay 45 is closed, indicator lamp 50 is shortcircuited, but

when relay 45 opens, a small current flows from power supply line 69through lamp 50, and thence through the solenoid valve andback to powerreturn line 71. This activates lamp 50. At the same time the operatingvoltage to lamp 49 is reduced to a level below the illuminationthreshold. Lamps 49 and 50 are mounted on circuit board 26, and may beobserved through small windows in the front of upper section 3.

Feeding back from the collector of transistor 56 is a positive feedbacksignal through high resistance resistor 57. The purpose of this feedbackis to provide a deadband for ON/OFF operation. A threshold forconduction of transistor 56 is initially set by adjustment of variableresistor 58 in null control 47. Once transistor 56 conducts, thefeedback signal through resistor 57 is summed in with the signalsprovided by amplifiers 53 and 54 to difference amplifier 55. Thisincreases the output of amplifier 55 and maintains transistor 56 abovethe operating threshold until the difference between the outputs ofamplifiers 53 and 54 is reduced to a voltage less than somepredetermined value. A resistance of about megoh'ms in resistor 57produces a deadband of about 0.6 to 0.9 millivolts as seen at the outputof probes 4 and 5. Capacitor 59 acts as a filter to slow down fast ON/OFF cycling rates. Resistors 60 and 61 and capacitors 62 and 63 provides adecoupling network between power supply 46 and the above mentionedamplifiers.

Circuit boards 25 and 26 are interconnected by 5 wires. These are theplus and minus 15 volt power lines and lines 64, 65 and 66. Line 64 is aground line which is connected to ground rod 6 and also the groundterminal of power supply 46.

Switch 52 is a three position device with ON, OFF, and CONTROLpositions. Actuation of switch 52 actuates two switches 52a and 52b tiedtogether as shown in FIG. '10. For normal operation switch 52 is in theCONTROL position. Selection of the ON position results in the removal ofcurrent from the solenoid valve and the substitution of resistor 67 forthe solenoid in the relay output circuit. Power supply 46 is a widelyavailable common device and preferably should be able to deliver about25 milliamperes of regulated DC current at plus and minus 15 volts asshown.

An example of an application of this invention to a papermaking processis illustrated in FIG. 9 wherein digested pulp is passed successivelythrough three wash tanks 85, 86 and 87 for removal of black liquorresidues. Thereafter the pulp will be bleached and then refined forpapermaking. The residues which are washed from the pulp comprise asodium sulfide component, other sodium compounds, organic acids andsugars. It is desirable from an economic viewpoint to recover a largeportion of the sodium sulfide, and this is accomplished by taking theeffluent from the first wash tank and passing it thourgh an evaporator.The sodium sulfide thus recovered may then be oxidized to produce freesulfer. It will be appreciated that the effluent from the second andthird wash tanks is quite low in black liquor residues and directrecovery of sodium sulfide therefrom is not economically feasible.Therefore washing solution from the third and second stages is cascadedforward to stage 1 as shown.

In addition to the wash tanks the system of FIG. 9 incorporates holdingtanks 88, 89 and 9t), recirculation pumps 91, 92 and 93, controllers 94,95 and 96, low level controls 101, 102 and 103, high level controls 104,105 and 106, and valves 108, 199, and 111. Controllers 94, 95 and 96 areconstructed as previously described with their solid state membranes 39made of a silver/silver sulfide composition asdescribed in copendingapplication Ser. No. 235,055.- Valves 108 through 111 are all solenoidoperated with valves 109, 108 and 111 respectively being actuated bycontrollers 94, 95 and 96. Each of the valves may also be actuated bythe high and low level controls as illustrated.

The washing solution within tank 88 is maintained by controller 94 at aconstant sodium sulfide concentration just below 10 grams per liter.This is accomplished as previuosly described by placing a sample ofwashing solution at the 10 gram concentration into reference tube 10.Whenever the concentration rises above 10 grams per liter, controller 94actuates valve 109 admitting relatively more pure second stage washingsolution into the first stage recirculation loop. Then as tank 88becomes filled, high level control 104 actuates valve 1 10 sending firststage washing solution to the evaporator.

The operation of the second and third stages is similar to the operationof the'first except for the control settings of controllers 95 and 96.Controller 95 maintains the second stage washing solution at aconcentration of 3 grams of sodium sulfide per liter of solution andmakes control corrections by opening valve 108 to admit third stagewashing solution into the second stage recirculation system. Controller96 actuates valve 111 to admit fresh water into the third stage wash asnecessary for maintenance of a concentration of 1 gram sodium sulfideper liter of washing solution. High level controls 105 and 106 and lowlevel controls 101, 102 and 103 performs a safety function only. Thuseither of controls 102 and 106 can open valve 108 to admit third stagewashing solution into the second stage recirculation system, butordinarily opening commands will come only from controller 95.Accordingly fresh water enters to cascade through the system and exitswith a constant sodium sulfide concentration of 10 grams per liter.

Alternative embodiments for the controller of this invention aredisclosed in FIG. 11 and 12. These embodiments are similar to thepreferred embodiment of FIG. 1 except for the capillary junction betweenthe process solution and the reference solution. In FIG. 11 the junctionis provided by a capillary passage 81 whereas in FIG. 12 the junction isprovided by a capillary passage 82. In both cases the reference tube 80is bonded in place. This contrasts with the tapered junction andremovable tube 10 of FIG. 1.

In still another embodiment the capillary junction may be made so largeas to permit a slow interchange of fluid between the reference chamberand the sample chamber. This alters the composition of the referencesolution and makes it in effect a sample of process solution averagedover some past period of time as determined by the fluid interchangerate. Such an arrangement is desirable in cases wherein the referencesolution has a tendency to deteriorate over a period of time, and it isnot convenient to shut down and change the reference solution.Accordingly the controller continues to replenish the process stream atan average rate equal to the average rate during some period in thepast. (Instantaneous replenishment follows an ON/OFF cycle in accordancewith the conduction cycle of transister 56) Thus there is compensationfor reference solution degradation but with an inherent slight longperiod drift.

Obviously other junctions such as a porous plug, a salt bridge, or anelectrically conductive reference tube wall may be employed. In anyevent it is important that the junction resistance be low relative tothe resistance of probes 4 and so as to maximize the sensitivity of thecontroller. For the preferred embodiment with the reference tube seatedby a seating force ranging from 1 to pounds, the junction resistanceranges from about 10 to 200 ohms. This compares with an individual proberesistance of about 7,500 ohms for a probe and made as described inapplication Ser. No. 235,055.

What is claimed is:

l. A process controller for regulation of the concentration of specificions in a stream of process fluid com- 6. a compartment within saidvessel for holding a reference solution in thermal equilibrium with thesurrounding process fluid within said vessel,

7. means for providing an electrical bridge between said process fluidand said reference solution,

8. a pair of identical probes each provided with a solid state membranesensitive to said specific ions; one probe extending into said vesselfor contact with said process fluid sample and the other probe extendinginto said vessel for contact with said ref erence solution, and

9. means connected to said probes for generation of a control signalrelated to the difference between the ion concentrations sensed by saidmembranes.

2. A process controller comprising an upper section and a lower sectionreleasably joined with a pressure seal therebetween, said upper sectioncomprising:

1. an enclosed housing,

2. an electronics package mounted within said housing and operative togenerate a chemical solution correction signal in response to a pair ofreference and measured input potentials, and

3. a pair of substantially identical specific ion sensors detachablymounted exteriorly below said housing and in electrical connection withsaid electronics package;

and said lower section comprising:

1. a first compartment for holding a reference solution,

2. a second compartment for holding a sample of a process solutionrequiring concentration correction in variable amount as determined bysaid correction signal,

3. means for providing an electrical bridge between ssid first andsecond compartments,

4. input and output connections for providing a continuous flow of saidprocess solution to said second compartment, and

5. means for dividing the process solution flowing into said inputconnection whereby a first portion thereof flows directly to said outputconnection and a second portion thereof flows through said secondcompartment;

said specific ion sensors extending downwardly whereby each reaches intoone of said compartments to generate said reference and measuredpotentials.

3. A process controller according to claim 2 wherein upper sectionfurther comprises a ground probe mounted exteriorly below said housing,said ground probe reaching upwardly for grounding connection with saidelectronics package and extending downwardly into said secondcompartment for grounding connection with said process solution.

4. A process controller according to claim 3 wherein said specific ionsensors are of identical construction including at their lower ends apair of solid state membranes sensitive to'the presence of said specificion in reference and process solutions contained within said first andsecond compartments.

5. A process controller according to claim 4 wherein said processcontroller is provided with means for upwardly admitting said processsolution to said second compartment at a point directly below thespecific ion sensor reaching into said compartment.

6. A process controller according to claim 4 wherein said specific ionsensors are mounted to said housing by means of threaded connections andelectrically connected to said electronics package by means of pinjacks.

7. A process controller according to claim 2 wherein said firstcompartment is a cylindrical tube.

8. A process controller according to claim 7 wherein said means forproviding an electrical bridge comprises a tapered circular wall inthebase of said lower section and a rough ground taper at the base ofsaid cylindrical tube.

9. A process controller according to claim 8 wherein said cylindricaltube is provided with a plurality of apertures in the region of saidrough ground taper for reduction of electrical resistance across saidelectrical bridge.

w. A process controller according to claim 7 wherein said upper sectionis provided with a circular groove for reception of the upper end ofsaid cylindrical tube and creation thereby of a condensate collectionchamber.

11. Apparatus according to claim 2 wherein said means for dividing theprocess solution is such that the said first portion of the processsolution is a major portion thereof and said lower section of theprocess controller further comprising means for re-combining said secondportion of the process solution with said first portion prior to exitthereof through the output connection.

12. A process controller according to claim 11 further comprising meansfor measuring the rate of flow of said second portion of processsolution and means for regulating said rate of flow.

13. A process controller for regulation of the concentration of specificions in a process solution comprising:

1. a vessel for holding a sample of said process solution,

2. input and output connections for providing a continuous flow of saidprocess solution through said vessel,

3. a compartment within said vessel for holding a reference solution inthermal equilibrium with said process solution,

4. means for providing an electrical bridge between siad processsolution and said reference solution,

5. a first probe extending into said compartment and operative togenerate a first electrical signal related to the concentration of saidspecific ions in said reference solution,

6. a second probe substantially identical to said first probe extendinginto said vessel and operative to generate a second electrical signalrelated to the concentration of said specific ions in said processsolution,

7. means connected to said first and second probes for generating athird electrical signal related to the difference between said first andsecond electrical signals,

8. means for generating a concentration correction signal when saidthird electrical signal exceeds a first predetermined magnitude, and

9. means for interrupting generation of said concentration correctionsignal when said third electrical signal falls below a secondpredetermined magnitude.

14. Process control apparatus comprising:

1. a vessel for holding a sample of a process solution,

2. input and output connections for providing a continuous flow of saidprocess solution through said vessel,

3. a compartment for holding a reference solution in electricalcommunication with the process solution in said vessel, 7

4. grounding means for establishing the potential of said processsolution as a ground potential,

5. a measuring probe comprising an ion sensing membrane and an outputconnection, said membrane being positioned for contact with processsolution within said vessel and said probe being operative to generatebetween said output connection and said membrane an electrical potentialrelated to the concentration of a predetermined specific ion within saidprocess solution,

6. a reference probe substantially identical to said measuring probecomprising an ion sensing membrane and an output connection, saidmembrane being positioned for contact with reference solution withinsaid compartment and said probe being operative to generate between saidoutput connection and said membrane an electrical potential related tothe concentration of a predetermined specific ion within said referencesolution,

7. first amplifying means for amplifying the potential between saidgrounding means and the output connection of said measuring probe,

8. second amplifying means for amplifying the potential between saidgrounding means and the output connection of said reference probe,

9. third amplifying means for amplifying the difference between theoutputs of said first and second amplifying means, and

10. means responsive to said third amplifying means for generating aprocess control signal for said process solution.

1. A process controller for regulation of the concentration of specificions in a stream of process fluid comprising:
 1. a container closedagainst the atmosphere,
 2. means for conducting said stream through saidcontainer,
 3. a vessel within said container for holding a sample ofsaid process fluid,
 4. means for continuous delivery of a small portionof said stream to said vessel,
 5. means for maintaining the processfluid within said vessel at a predetermined level and continuouslyreturning excess process fluid to said stream,
 6. a compartment withinsaid vessel for holding a reference solution in thermal equilibrium withthe surrounding process fluid within said vessel,
 7. means for providingan electrical bridge between said process fluid and said referencesolution,
 8. a pair of identical probes each provided with a solid statemembrane sensitive to said specific ions; one probe extending into saidvessel for contact with said process fluid sample and the other probeextending into said vessel for contact with said reference solution, and9. means connected to said probes for generation of a control signalrelated to the difference between the ion concentrations sensed by saidmembranes.
 2. a second compartment for holding a sample of a processsolution requiring concentration correction in variable amount asdetermined by said correction signal,
 2. input and output connectionsfor providing a continuous flow of said process solution through saidvessel,
 2. means for conducting said stream through said container, 2.input and output connections for providing a continuous flow of saidprocess solution through said vessel,
 2. A process controller comprisingan upper section and a lower section releasably joined with a pressureseal therebetween, said upper section comprising:
 2. an electronicspackage mounted within said housing and operative to generate a chemicalsolution correction signal in response to a pair of reference andmeasured input potentials, and
 3. A process controller according toclaim 2 wherein upper section further comprises a ground probe mountedexteriorly below said housing, said ground probe reaching upwardly forgrounding connection with said electronics package and extendingdownwardly into said second compartment for grounding connection withsaid process solution.
 3. a compartment within said vessel for holding areference solution in thermal equilibrium with said process solution, 3.a vessel within said container for holding a sample of said processfluid,
 3. a compartment for holding a reference solution in electricalcommunication with the process solution in said vessel,
 3. means forproviding an electrical bridge between ssid first and secondcompartments,
 3. a pair of substantially identical specific ion sensorsdetachably mounted exteriorly below said housing and in electricalconnection with said electronics package; and said lower sectioncomprising:
 4. input and output connections for providing a continuousflow of said process solution to said second compartment, and 4.grounding means for establishing the potential of said process solutionas a ground potential,
 4. means for continuous delivery of a smallportion of said stream to said vessel,
 4. means for providing anelectrical bridge between siad process solution and said referencesolution,
 4. A process controller according to claim 3 wherein saidspecific ion sensors are of identical construction including at theirlower ends a pair of solid state membranes sensitive to the presence ofsaid specific ion in reference and process solutions contained withinsaid first and second compartments.
 5. A process controller according toclaim 4 wherein said process controller is provided with means forupwardly admitting said process solution to said second compartment at apoint directly below the specific ion sensor reaching into saidcompartment.
 5. a first probe extending into said compartment andoperative to generate a first electrical signal related to theconcentration of said specific ions in said reference solution,
 5. meansfor maintaining the process fluid within said vessel at a predeterminedlevel and continuously returning excess process fluid to said stream, 5.a measuring Probe comprising an ion sensing membrane and an outputconnection, said membrane being positioned for contact with processsolution within said vessel and said probe being operative to generatebetween said output connection and said membrane an electrical potentialrelated to the concentration of a predetermined specific ion within saidprocess solution,
 5. means for dividing the process solution flowinginto said input connection whereby a first portion thereof flowsdirectly to said output connection and a second portion thereof flowsthrough said second compartment; said specific ion sensors extendingdownwardly whereby each reaches into one of said compartments togenerate said reference and measured potentials.
 6. a reference probesubstantially identical to said measuring probe comprising an ionsensing membrane and an output connection, said membrane beingpositioned for contact with reference solution within said compartmentand said probe being operative to generate between said outputconnection and said membrane an electrical potential related to theconcentration of a predetermined specific ion within said referencesolution,
 6. a compartment within said vessel for holding a referencesolution in thermal equilibrium with the surrounding process fluidwithin said vessel,
 6. a second probe substantially identical to saidfirst probe extending into said vessel and operative to generate asecond electrical signal related to the concentration of said specificions in said process solution,
 6. A process controller according toclaim 4 wherein said specific ion sensors are mounted to said housing bymeans of threaded connections and electrically connected to saidelectronics package by means of pin jacks.
 7. means connected to saidfirst and second probes for generating a third electrical signal relatedto the difference between said first and second electrical signals, 7.means for providing an electrical bridge between said process fluid andsaid reference solution,
 7. first amplifying means for amplifying thepotential between said grounding means and the output connection of saidmeasuring probe,
 7. A process controller according to claim 2 whereinsaid first compartment is a cylindrical tube.
 8. second amplifying meansfor amplifying the potential between said grounding means and the outputconnection of said reference probe,
 8. a pair of identical probes eachprovided with a solid state membrane sensitive to said specific ions;one probe extending into said vessel for contact with said process fluidsample and the other probe extending into said vessel for contact withsaid reference solution, and
 8. means for generating a concentrationcorrection signal when said third electrical signal exceeds a firstpredetermined magnitude, and
 8. A process controller according to claim7 wherein said means for providing an electrical bridge comprises atapered circular wall in the base of said lower section and a roughground taper at the base of said cylindrical tube.
 9. means forinterrupting generation of said concentration correction signal whensaid third electrical signal falls below a second predeterminedmagnitude.
 9. third amplifying means for amplifying the differencebetween the outputs of said first and second amplifying means, and 9.means connected to said probes for generation of a control signalrelated to the difference between the ion concentrations sensed by saidmembranes.
 9. A process controller according to claim 8 wherein saidcylindrical tube is provided with a plurality of apertures in the regionof said rough ground taper for reduction of electrical resistance acrosssaid electrical bridge.
 10. means responsive to said third amplifyingmeans for generating a process control signal for said process solution.10. A process controller according to claim 7 wherein said upper sectionis provided with a circular groove for reception of the upper end ofsaid cylindrical tube and creation thereby of a condensate collectionchamber.
 11. Apparatus according to claim 2 wherein said means fordividing the process solution is such that the said first portion of theprocess solution is a major portion thereof and said lower section ofthe process controller further comprising means for re-combining saidsecond portion of the process solution with said first portion prior toexit thereof through the output connection.
 12. A process controlleraccording to claim 11 further comprising means for measuring the rate offlow of said second portion of process solution and means for regulatingsaid rate of flow.
 13. A process controller for regulation of theconcentration of specific ions in a process solution comprising: 14.Process control apparatus comprising: